Quarterly Results of Operations (Unaudited) - Schedule of Quarterly Financial Information (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 25, 2019 USD ($) restaurant $ / shares $ / restaurant shares	Sep. 25, 2019 USD ($) restaurant $ / shares $ / restaurant shares	Jun. 26, 2019 USD ($) restaurant $ / shares $ / restaurant shares	Mar. 27, 2019 USD ($) restaurant $ / shares $ / restaurant shares	Dec. 26, 2018 USD ($) restaurant $ / shares $ / restaurant shares	Sep. 26, 2018 USD ($) restaurant $ / shares $ / restaurant shares	Jun. 27, 2018 USD ($) restaurant $ / shares $ / restaurant shares	Mar. 28, 2018 USD ($) restaurant $ / shares $ / restaurant shares	Dec. 25, 2019 USD ($) restaurant $ / shares shares	Dec. 26, 2018 USD ($) restaurant $ / shares shares	Dec. 27, 2017 USD ($) $ / shares shares
Selected Financial Data
Total revenue | $	$ 107,546	$ 112,067	$ 113,740	$ 108,977	$ 106,261	$ 112,178	$ 111,633	$ 105,756	$ 442,330	$ 435,828	$ 401,701
(Loss) income from operations | $	5,336	10,118	20,580	2,292	(30,990)	9,492	7,589	4,448	38,326	(9,461)	6,824
Provision (benefit) for income taxes ($) | $	728	2,940	5,665	349	(8,410)	2,388	865	1,949	9,682	(3,208)	497
Net income (loss) | $	$ 3,498	$ 6,402	$ 14,087	$ 913	$ (23,410)	$ 6,835	$ 5,052	$ 2,529	$ 24,900	$ (8,994)	$ 8,619
Net income (loss) per share:
Basic (usd per share) | $ / shares	$ 0.10	$ 0.18	$ 0.37	$ 0.02	$ (0.60)	$ 0.18	$ 0.13	$ 0.07	$ 0.68	$ (0.23)	$ 0.22
Diluted (usd per share) | $ / shares	$ 0.10	$ 0.18	$ 0.37	$ 0.02	$ (0.60)	$ 0.17	$ 0.13	$ 0.06	$ 0.67	$ (0.23)	$ 0.22
Weighted average shares used in computing net income (loss) per share:
Basic (shares) | shares	34,503,722	35,859,502	37,939,912	38,633,702	38,751,522	38,602,658	38,482,074	38,465,208	36,739,209	38,574,553	38,453,347
Diluted (shares) | shares	35,242,122	36,397,368	38,580,722	39,496,436	38,751,522	39,205,090	39,043,434	38,987,351	37,441,503	38,574,553	39,086,676
Number of Restaurants | restaurant	4				4				4	4
Selected Operating Data
Average unit volume (AUV) (usd per restaurant) | $ / restaurant	1,931	1,978	1,934	1,838	1,785	1,891	1,890	1,791
Company-operated
Weighted average shares used in computing net income (loss) per share:
Number of Restaurants | restaurant	195	201	200	211	213	212	211	212	195	213
Comparable restaurant sales growth (%)
Comparable restaurant sales growth (%)	4.30%	1.60%	0.40%	1.50%	3.70%	2.00%	(1.60%)	(2.00%)
Franchised
Weighted average shares used in computing net income (loss) per share:
Number of Restaurants | restaurant	287	284	284	273	271	271	269	268	287	271
Comparable restaurant sales growth (%)
Comparable restaurant sales growth (%)	3.60%	0.60%	0.90%	3.20%	5.10%	3.00%	(0.30%)	(0.40%)
System-wide
Weighted average shares used in computing net income (loss) per share:
Number of Restaurants | restaurant	482	485	484	484	484	483	480	480	482	484
Comparable restaurant sales growth (%)
Comparable restaurant sales growth (%)	3.90%	1.10%	0.70%	2.40%	4.40%	2.60%	(0.90%)	(1.10%)